Rename some helpers to middleware

We should rename the following `Helpers` to be `Middleware` instead, and follow the style of `CacheMiddleware`:

- `BasicAuth`
- `BearerAuth`
- `BodyLimit`
- `CORS`
- `Compress`
- `RequestId`
- `Logger`

This would be a breaking change, but since we're still in early stages, I don't think this is a huge concern.